Microsoft unveiled Windows Phone 8.1 on Wednesday, and confirmed reports that the new OS includes Microsoft's first voice-activated digital assistant, called Cortana.

Windows Phone 8.1 will include a voice-activated digital assistant called Cortana.

Windows Phone 8.1 won't be generally available for several months -- it's slated to run new phones shipping in late April or early May, according to Joe Belfiore, corporate vice president of Microsoft's operating systems group.

Belfiore spoke at the opening of Microsoft's Build 2014 developer conference, which was webcast.

Another major new feature in WP 8.1 is a shape writing capability now commonly available in some other smartphones and via add-on apps. Belfiore demonstrated the shape writing by swiping his finger through a keyboard without lifting it between letters.

Cortana will first appear in beta.

The digital assistant hits the smartphone scene three years after Siri was launched on the iPhone 4S with iOS 5 and two years after Google Now for Android 4.1 smartphones was launched in 2012.

But Belfiore argues that Cortana is better than Siri, calling it the "first truly personal digital assistant who learns about me and the people that matter to me most."

Belfiore introduced the female voice of Cortana and said the digital assistant can also be contacted via text input.

During several demonstrations, he showed how Microsoft tried to mirror the way a real-life personal assistant would manage a person's calendar or relationships. By using Microsoft's Bing search engine, Cortana "knows everything about the world" as well as information contained on a user's phone, he said.

In one demonstration, he asked Cortana to set up a dental appointment on Aug. 1 at 10 a.m., but Cortana replied that he was expected to pick somebody up at the airport at the same time. In another demo, he asked Cortana to remind him to ask a friend how her puppy was doing when she called the next time, and the reminder appeared in text on the phone's display when the phone call came from her.

Belfiore was subject to a couple of demo mishaps with Cortana. The first came when he asked Cortana to pick the second of two Mexican restaurants to call that were displayed on the phone and Cortana apparently started to call the first in the list. Calling the mishap a "minor bug," he added, "We're launching Cortana as a beta while we train the service on the back end, with lots of voice utterances to improve speech recognition."

But Cortana did use its intelligence to find several four-star Mexican restaurants in Palo Alto, Calif., after Belfiore asked her to find the best ones. And Cortana was also able to show four out of the group that take reservations.

In another mishap, he asked Cortana to convert a temperature from Farenheit to Celsius, which worked, but when he asked Cortana to covert that temperature reading to the Kelvin temperature scale, Cortana twice repeated, "I converted to Celsius ..."

Belfiore called that mishap "the vagaries of a service that's live and up and down ... We tried to make her smart."

Cortana can also recognize a user's closest connections, based on the frequency they are contacted and other usage patterns, Belfiore said. The list of closest personal connections can help Cortana decide whether to put through a late night call. The close personal contact list can be customized by the user, based on Cortana's suggestions.

Belfiore saved his riskiest demo for last, in which he used fingers on a smartphone's virtual keyboard to show how shape writing works. Only one mispelling occurred in the complex sentence he typed: "Being onstage at Build is simultaneously delightful and horrifying. Of course I love to hear from developers, but occasionally techies can be intimidating."

Patrick Moorhead, an analyst at Moor Insight & Strategy, said Cortana's addition to Windows Phone "is impressive but so was Siri and Google Now when it was demoed on stage, so I'm skeptical that it can make a different in Windows Phone sales."